2 pins is mechanically weak, 4 pins means more options for routing traces past / through the PCBreal-estate under the switch, some switches with the same footprint will be SPDT and need atleast 3 pins anyway. There is a cultural bias in favour of rectangles over triangles/hexagons in engineering too (for instance the standard QWERTY keyboard can be more logically made fromhexagonal keys rather than square, yet hexagonal key switches are never seen)

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Yet another reason to have two pins per terminal is that the switch can then double as two jumpers. This can allow a single-sided PC board design and thus save money over a two-sided design. Traces traveling one way can use the switches to jump over traces running perpendicular to that direction.